Amzel, L. M. | bioRxiv Science

bioRxiv2022

Serine Racemase is a Cysteine Racemase and Physiologic Down Regulator of Insulin Promoter Methylation

D-amino acids are being recognized in mammals as important molecules with function. This is a first identification of endogenous D-cysteine in mammalian pancreas. D-cysteine is synthesized by serine racemase (SR) and SR-/- mice produce 6-10 fold higher levels of insulin in the pancreas and plasma including higher glycogen and ketone bodies in the liver. The excess insulin is stored as amyloid in secretory vesicles and exosomes. In glucose stimulated insulin secretion studies in mouse and human islets, equimolar amount of D-cysteine showed higher inhibition of insulin secretion compared to D-serine, another closely related stereoisomer synthesized by SR. In mouse models of diabetes (STZ and NOD) and human pancreas, the diabetic state showed increased expression of D-cysteine compared to D-serine followed by increased expression of SR. SR-/- mice show decreased cAMP in the pancreas followed by reduced phosphorylation of CREB (S133), lower DNA methyltransferase enzymatic and promoter activities resulting in decreased methylation of the Ins1 promoter. D-cysteine is efficiently metabolized by D-amino acid oxidase and transported by ASCT2 and Asc1. Dietary supplementation with methyl donors restored the high insulin levels and low DNMT enzymatic activity in SR-/- mice. Our data show that endogenous D-cysteine in the mammalian pancreas is a regulator of insulin secretion. HighlightsO_LISerine Racemase also functions as a cysteine racemase. C_LIO_LILack of Serine Racemase results in significantly high levels of insulin in the pancreas, plasma and larger islets. C_LIO_LID-cysteine shows greater inhibition of insulin secretion compared to D-serine. C_LIO_LIEndogenous D-cysteine signals via cyclic AMP that mediates downstream CREB-DNMT1 interaction. C_LIO_LICREB-DNMT1 interaction results in hypomethylation of Ins1 promoter that can be rescued by high methyl donor dietary supplementation rescuing high insulin levels. C_LI

biochemistry↗

bioRxiv2022

Structural insights into sodium/iodide symporter (NIS) substrate binding and specificity

The sodium/iodide symporter (NIS) is the essential plasma membrane protein that mediates active iodide (I-) transport into the thyroid gland, the first step in the biosynthesis of the thyroid hormones--the master regulators of intermediary metabolism. NIS couples the inward translocation of I- against its electrochemical gradient to the inward transport of Na+ down its electrochemical gradient. For nearly 50 years before its molecular identification, NIS was already the molecule at the center of the single most effective internal radiation cancer therapy ever devised: radioiodide (131I-) treatment for thyroid cancer. Mutations in NIS cause congenital hypothyroidism, which must be treated immediately after birth to prevent stunted growth and cognitive deficiency. To date, the structure of NIS has been unknown. Here, we report three structures of rat NIS, determined by single-particle cryo-electron microscopy (cryo-EM): one with no substrates bound, one with 2 Na+ and 1 I- bound, and one with 1 Na+ and the oxyanion perrhenate bound. Structural analyses, functional characterization, and computational studies reveal the substrate binding sites and residues key for transport activity. Our results yield insights into how NIS selects, couples, and translocates anions--thereby establishing a framework for understanding NIS function--and into how it transports different substrates with different stoichiometries and releases substrates from its substrate-binding cavity into the cytosol.

biophysics↗